There is much art on flame-retardants, but little on smoke-retarders or suppressants. A paper entitled SMOKE GENERATION FROM THE BURNING OF SOME POLYMERIC MATERIALS by Brauman et al., given at the 32nd Annual Technical Conference of the Society of Plastic Engineers held in San Francisco in May 1974 discusses the problem and the effect of several smoke deterrents, but does not suggest the invention. The use of zinc borate (2ZnO.3B.sub.2 O.sub.3.3.5 H.sub.2 O) as a flame-retardant is well known in the industry (Plastics Technology 20 (8) pp. 41-47 (1974).
Patents on smoke retardants include, for example, Frisch U.S. Pat. No. 3,725,319 and Doerge et. al. U.S. Pat. Nos. 3,746,664 and 3,758,638, but these include no suggestion of the invention.